In bacterial DNA replication, which DNA polymerase is the primary replicative enzyme responsible for leading and lagging strand synthesis at the replication fork?

Introduction / Context:
Bacterial cells replicate their genomes rapidly and accurately. Multiple DNA polymerases exist, but only one serves as the main replicative enzyme at the fork. Identifying DNA polymerase III as the principal enzyme is a foundational concept in molecular genetics and microbiology.

Given Data / Assumptions:

• The organism category is bacteria (for example, Escherichia coli).
• We consider the replication fork with leading and lagging strands.
• DNA polymerases differ in roles: replication, repair, primer removal.

Concept / Approach:
In E. coli, DNA polymerase III holoenzyme is a multi-subunit complex (including core polymerase, beta clamp, clamp loader) that performs high-processivity DNA synthesis on both strands. DNA polymerase I has 5′→3′ exonuclease activity to remove RNA primers and fills short gaps, while DNA polymerase II functions mainly in DNA repair and restart. Therefore, Pol III is the correct answer.

Step-by-Step Solution:
List functions: Pol III (replication), Pol I (primer removal and gap filling), Pol II (repair).Identify the enzyme with highest processivity at forks: Pol III with beta clamp.Recognize that both leading and lagging strand synthesis are coordinated by Pol III dimers.Conclude Pol III is the primary replicative polymerase.

Verification / Alternative check:
Genetic and biochemical experiments show that mutations in Pol III subunits are lethal or strongly replication-defective, while Pol I mutants specifically impair primer removal, confirming functional distinctions.

Why Other Options Are Wrong:
DNA polymerase I: essential for primer removal and short-patch synthesis, not the main processive replicase.DNA polymerase II: repair-associated; not the fork replicase.All of these: overbroad; only Pol III is the primary replicative enzyme.

Common Pitfalls:
Assuming all polymerases replicate chromosomal DNA equally. Processivity and accessory factors (sliding clamp) define the true replicase.

Final Answer:
DNA polymerase III